Presently known intruder detectors making use of a straight beam of light or infrared rays generally utilize a parabolic mirror as an optical element to increase the density of a collected beam in a projecting or an accepting direction, the focal position of the mirror being occupied by a photo-electric transducer.
A passive type intruder detector compares infrared energy radiated from an intruder's body with infrared energy radiated from a background material object and generates an output signal when there is a difference above a predetermined level. An intruder detecting device of this type is disclosed in Japanese Laid-Open Utility Model Publication No. 97,534 of 1980 and Japanese Laid-Open Patent Publication No. 143,694 of 1980. Because the visual field of this infrared detector is a maximum angle of 90 degress, any extension of a detecting region beyond said visual field is dependent upon the optical elements in the system. In the device disclosed by said Laid-Open Patent Publication No. 143,694 of 1980, the sensing region may be extended to an angle of approximately 180 degrees by combining a parabolic mirror with an alignment structure of plane mirrors. However, where the device is mounted on the ceiling of a structure, the device has no sensitivity in the backward direction or generally in the downward direction. It should also be noted that the alignment structure of the plane mirrors is arranged into steps of mirrors, and therefore, it is different to incline each of the divided plane mirrors with respect to the optical axis of the parabolic mirror so as to obtain a larger number of directions sensitive to intruders, and to equalize all the effective projecting areas of the divided mirrors onto the parabolic mirror. Another difficult problem in the prior device is that each of the outer distributed plane mirrors cannot project their whole mirror surface onto the parabolic mirror, and therefore a substantial difference exists between the sensitivity of the inner and the outer mirrors.
It is an object of the invention to solve the above technical problems by providing a simply constructed optical system for an intruder detector in which the detecting region expands an angle of 360 degrees, and in which no substantial difference in sensitivity is present for the various detecting directions.